Selma Blair's Son Asked If She Was Dying After MS Diagnosis: 'I Always Want Him to Feel Safe'

Selma Blair‘s 7½-year-old son Arthur Saint had one big question after his mother told him she had multiple sclerosis.

“After the MRI, I said, ‘I have something called multiple sclerosis,’ and he almost cried and said, ‘Will it kill you?’ ” the actress told Robin Roberts in a Tuesday sit-down for Good Morning America — her first television interview since revealing her MS diagnosis in October.

“And I said, ‘No. I mean, we never know what kills us, Arthur. But this is not the doctor telling me I’m dying,’ ” recalled Blair, 46. “And he was like, ‘Oh. Okay!’ “

Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Parents newsletter.

Selma Blair Opens Up in First Interview Since MS Revelation: I “Cried with Relief” After Diagnosis

Blair shared with Roberts that she didn’t have a problem “at all” being honest with her only child about her condition, especially considering he sensed on his own that things had changed.

“I always want him to feel safe, never responsible for me, but he had already seen that I was falling and doing [different] things, and I was always laughing,” she said.

“And he’d imitate me,” added the Cruel Intentions actress. “I’d be like, ‘That’s fine, but don’t do that [outside] of the house. People will think you’re a jerk.’ ”

Blair said she experienced symptoms for some time before learning she had MS but was “giving it everything to seem normal” around her son initially — including “self-medicating.”

“There were times when I couldn’t take it and I was really struggling with how I’m going to get by in life,” she admitted, noting she was “not being taken seriously by doctors.”

Recounted the Can’t Hardly Wait actress, “I dropped my son off at school a mile away and before I got home I’d have to pull over and take a nap … it was killing me. And so when I got the diagnosis, I cried with relief.”

She has also learned along the way that it’s okay to take a day off if she needs it. “My son gets it and now I’ve learned to not feel really guilty,” Blair explained.